too much moves in armature - pose
You've parented the the ribs to the armature, so accidentally the arm acts on the ribs and you need to correct that:

Select the arm to know its name:



Select the ribs, go in the Properties panel &gt; Object Data &gt; Vertex Groups, find the vertex groups that has the same name, click on Remove:


The lower arm influences the ribs as well so do the same. You can also do it in Weight Paint mode.